Primary exergy efficiency—Effect of system efficiency environment to benefits of exergy savings

• An exergy-based energy efficiency method called PeXa is developed.
• PeXa combines exergy analysis and primary energy analysis.
• PeXa uses exergy analysis inside a process and primary energy based factors for the surrounding society.
• In some cases exergy analysis and PeXa will give different results assuming that the objective is to consider the primary energy effects of society.

Improving energetic performance is a key factor in making societies more sustainable. One way to analyze energetic performance is to use methods based on the second law of thermodynamics. Exergy analysis is such a method. With exergy analysis thermodynamic losses of the studied system can be found. For a specific process decreasing the exergy losses decreases the need for exergy inputs and production costs. Exergy analysis can also be used to analyze the life cycle of a process or product, but then it is necessary to model the total production system. For this reason, it is important to have efficiency analysis methods that can simultaneously analyze the studied system or process and the surrounding environment around this system. The objective of this article is to present such a method where the whole energy chain needs not to be modeled, but still the effect of an energy improvement or change in a studied process can be analyzed with respect to the whole energy chain. This method is called PeXa and it combines exergy analysis and primary energy analysis. In this work we show that also the system environment affects the benefit of exergy savings in the system level depending what production does this exergy saving replace. A district heating (DH) network with different DH producing units having different exergy efficiencies is used to show the concept. It is shown that in some cases basic exergy analysis and PeXa will give different results assuming that the objective is to consider the primary energy effects of society. By considering this broader concept of environment in exergy analysis companies and societies can direct limited resources into investments that maximize primary exergy savings.